In the present paper, the case of the brown algae Saccharina japonica from Aniva Bay (Sea of Okhotsk, Sakhalin Island) was investigated by a thermogravimetric analysis up to 700°C at different atmospheres. The elemental composition, lower heating value, ash content, and biochar yield of the algae were examined. The analysis showed that carbohydrates like alginate, mannitol, fucoidan, and laminarin were decomposed between 250-350°C, while proteins and lipids were burned out between 500-550°C.